This invention relates to a coating for a reflective surface, and, more particularly, to a coating which provides a uniform intensity in a polarized beam reflected from the reflective surface of a rotating polygon mirror over a wide range of incident angles and over a wide range of beam wavelengths.
Many conventional raster optical scanners utilize a multi-faceted rotating polygon mirror as the scanning element. A collimated beam of light of a single wavelength strikes the mirrored surface of the facets of the rotating polygon which causes the reflected light to revolve about an axis near the center of rotation of the rotating polygon and scan a straight line. This reflected light can be utilized as a scanning beam to scan a document at the input end of an imaging system or can be used as an imaging beam to imprint upon a photosensitive medium in the output mode.
The rotating polygon mirror of a raster output scanning system is typically formed of a reflective metal, such as aluminum. The aluminum substrate of the mirror is machined and polished to form flat reflective facets on the outside of a polygon. The aluminum facets are either left uncoated or, more commonly, given a dielectric material coating, such as silicon dioxide (SiO.sub.2), to protect the mirror surface from oxidation and damage.
Aluminum tends to oxidize over time into the less reflective aluminum oxide (Al.sub.2 O.sub.3). The mirrored surface of aluminum is sensitive to particle damage due to scratching and particularly since the polygon mirror is rotated at high speeds and occasionally cleaned. Hence, the reflective facets of the aluminum polygon mirror typically have a half wave optical thickness coating of silicon dioxide (SiO.sub.2) for protection.
The reflectance of a mirrored surface varies with the angle of incidence of the light beam. The reflectance of the mirrored surface also varies with the wavelength of the incident light beam.
In raster optical scanners, it is essential that the intensity of the scanning beam be accurately controlled at the scan line for scanning or imaging. A uniform intensity is required for the beam reflected from the facets of the rotating polygon mirror for precise imaging and scanning along the scan line and from scan line to scan line. This uniform intensity is important for gray scale printing, for example. The more uniform the intensity of the output power of the imaging beam, the more uniform the print pattern across the printed page will be. This uniform intensity is also important for scanning of an input document.
Moreover, as the polygon mirror of the raster optical scanner rotates, the angle of the incident beam striking the reflective surface of a mirror facet will vary as will the angle of reflection of the reflected beam from the mirror facet. Therefore, the intensity of the incident beam must be uniform over a wide range of angles as the beam strikes the rotating polygon mirror facet and is reflected across the scan line.
It is an object of this invention to provide a uniform intensity for a incident beam over a wide range of angles as the beam strikes the rotating polygon mirror facet and is reflected across the scan line.
It is another object of this invention to provide a uniform intensity for a incident beam over a wide range of wavelengths as the beam strikes the rotating polygon mirror facet and is reflected across the scan line.